Hibernate集合加载策略问题-优快云博客

本文链接：https://blog.youkuaiyun.com/logicigam/article/details/83367172

作者在使用BlazeDS与Hibernate集成时遇到延迟加载问题，导致查询结果重复。通过调整Hibernate配置，采用子查询方式成功解决问题。

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最近在做一个Flex项目，后台spring+hibernate+blazeDS，由于blazeDS不支持延迟加载，逼得我非得把延迟加载关掉，由于我用的是注释配置，我就直接在EJB3的@ManyToMany注解里加了fetch=FetchType.EAGER，详细配置如下


User.java

	@ManyToMany(cascade = { CascadeType.PERSIST,   CascadeType.MERGE },fetch=FetchType.EAGER)
	//多对多定义.
	@JoinTable(name = "USERS_ROLES", joinColumns = { @JoinColumn(name = "USER_ID") }, inverseJoinColumns = { @JoinColumn(name = "ROLE_ID") })
	//集合按id排序.
	@OrderBy("id")
	//集合中对象的id的缓存.
	@Cache(usage = CacheConcurrencyStrategy.READ_WRITE)
	public Set<Role> getRoles() {
		return roles;
	}

运行之后发现一个奇怪现象，查询出来的结果每个对象都重复了3份儿，到控制台看生成的SQL语句，发现是用的left outer join，


Hibernate: 
    select
        this_.id as id2_2_,
        this_.email as email2_2_,
        this_.login_name as login3_2_2_,
        this_.name as name2_2_,
        this_.password as password2_2_,
        roles2_.user_id as user1_4_,
        role3_.id as role2_4_,
        role3_.id as id1_0_,
        role3_.name as name1_0_,
        auths4_.role_id as role1_5_,
        authority5_.id as authority2_5_,
        authority5_.id as id0_1_,
        authority5_.display_name as display2_0_1_,
        authority5_.name as name0_1_ 
    from
        users this_ 
    left outer join
        users_roles roles2_ 
            on this_.id=roles2_.user_id 
    left outer join
        roles role3_ 
            on roles2_.role_id=role3_.id 
    left outer join
        roles_authorities auths4_ 
            on role3_.id=auths4_.role_id 
    left outer join
        authorities authority5_ 
            on auths4_.authority_id=authority5_.id 
    order by
        role3_.id asc,
        authority5_.id asc

越看越不对，拷到mysql控制台执行结果还是重复~看来干脆就是SQL语句的问题，左外连接没有过滤结果集。查了查Hibernate手册，转而用


@Fetch(FetchMode.SUBSELECT)
@LazyCollection(LazyCollectionOption.FALSE)

代替原来的fetch=FetchType.EAGER，使用子查询加载集合，测试成功，看来Hibernate也有靠不住的时候，谁能告诉我怎么用left outer join得到正确的结果？？？毕竟用了子查询的话会多发几条SQL语句，还是看着不爽。